1. Field of the Invention
The present invention relates to a motor apparatus without a position sensor or a speed sensor, and a control method therefor.
2. Description of the Related Art
Conventionally, a motor whose operations are controlled at a given position instruction (phase angle instruction) and speed instruction (rotation speed instruction) has been well known. This type of motor is normally provided with a sensor for detecting the position or speed of a rotor, and a feedback system is configured to have output of the sensor follow an instructed value.
However, if the above mentioned sensor is provided, it is difficult to reduce the cost of or downsize a motor. Additionally, depending on the environment of a motor, it is difficult to provide the above mentioned sensor. Therefore, a control method of estimating the position or speed according to other parameters without providing a sensor, and realizing the feedback system based on the estimated value has been suggested and used. The control method is hereinafter referred to as a xe2x80x9csensorless control methodxe2x80x9d. The sensorless control method is described in, for example, xe2x80x9cBack EMF Estimation-Based Sensorless Salient-Pole Brushless DC Motor (by Takaharu Takeshita, et al. in the Institute of Electrical Engineers of Japan, Vol.117-D No.1 published in 1997, on pages 98 to 104)xe2x80x9d. In the method described in this research paper (hereinafter referred to as the xe2x80x9cpaper Axe2x80x9d), the position and speed of a rotor are estimated based on an externally given instruction value and the detected value of an actual motor current, and feedback control is carried out using the values.
However, in the sensorless control method, a parameter relating to the operations of a motor is detected, and the position, speed, etc. of the rotor are estimated using the detected values. For example, in the method described in the xe2x80x9cpaper Axe2x80x9d, the position, etc. of a rotor is estimated based on the detected current. That is, the data for use in estimating the position, etc. has not been collected in the initial stages of operations. Thus, in the sensorless control method, a motor drive signal is often generated based on inappropriate position data in the early stages of operations. If the motor drive signal is generated based on the inappropriate position data, the efficiency of a motor is reduced.
The motor drive signal converges into an appropriate state soon or later by the feedback system although it is in an inappropriate state in the early stages of operations. However, before it converges into the appropriate state, the efficiency of the motor is low. Therefore, there has been the problem with the sensorless control method in the environment where there is a strict demand for a shortest possible time required to obtain a desired characteristic after a motor begins to be driven. Practically, there is the problem when the sensorless control method is adopted for a motor provided in a compressor of an air-conditioner of a car, for example. That is, since an air-conditioner for a car is often required to quickly condition the temperature in a car, low efficiency of a motor in the early stages of operations is not desired.
Additionally, the technology of estimating the initial position of a rotor in the sensorless control method is described in, for example, xe2x80x9cInitial Rotor Position Estimation of Sensorless Salient-Pole Brushless DC Motor (by Takaharu Takeshita, et al. in the Institute of Electrical Engineers of Japan Vol.116-D No.7 published in 1996, on pages 736 to 742)xe2x80x9d. However, in the method described in this paper, only the initial position is estimated, and there is the possibility that a wrong result is derived.
The present invention aims at improving the efficiency in the early stages of operations of the motor apparatus under the sensorless control.
The motor apparatus according to the present invention includes: a synchronous motor having a rotor; a controller for generating a control signal based an instruction value for instructing the operation of the synchronous motor, a current supplied to the synchronous motor, and the rotor position estimated based on the current; and a drive unit for driving the synchronous motor according to the control signal. The controller supplies a predetermined current pattern when the synchronous motor begins to be driven, and then generates the control signal using the rotor position obtained due to supplying the predetermined current pattern as an initial rotor position.
In this motor apparatus, the position of a rotor of the synchronous motor (that is, the phase of the rotor) is set in the position uniquely corresponding to the predetermined current pattern if it is supplied to the synchronous motor. Therefore, if the rotor position corresponding to the predetermined current pattern is used as the initial rotor position, then the actual rotor position matches the estimated position when the motor apparatus begins to be driven. As a result, the motor operation can be performed with high efficiency immediately after it begins to be driven.
The controller can be designed to start generating the control signal using the rotor position due to the predetermined current pattern as the initial rotor position when the predetermined current pattern becomes stable at a constant level. With this configuration, the time required to set the initial rotor position can be set to the minimal value.